the
partial pressures equals that of the atmosphere. Both components come
over in a constant proportion until one disappears; it is then
necessary to raise the temperature in order to distil the residue. The
composition of the distillate is determinate (by Avogadro's law) if
the molecular weights and vapour pressure of the components at the
temperature of distillation be known. If M1, M2, and P1, P2 be the
molecular weights and vapour pressures of the components A and B, then
the ratio of A to B in the distillate is M1P1/M2P2. Although, as is
generally the case, one liquid (say A) is more volatile than the other
(say B), i.e. P1 greater than P2, if the molecular weight of A be much
less than that of B, then it is obvious that the ratio M1P1/M2P2 need
not be very great, and hence the less volatile liquid B would come
over in fair amount. These conditions pertain in cases where
distillation with steam is successfully practised, the relatively high
volatility of water being counterbalanced by the relatively high
molecular weight of the other component; for example, in the case of
nitrobenzene and water the ratio is 1 to 5. In general, when the
substance to be distilled has a vapour pressure of only 10 mm. at 100
deg. C., distillation with steam can be adopted, if the product can be
subsequently separated from the water.

When distilling a mixture of partially miscible components a
distillate of constant composition is obtained so long as two layers
are present, i.e. A dissolved in B and B dissolved in A, since both of
these solutions emit vapours of the same composition (this follows
since the same vapour must be in equilibrium with both solutions, for
if it were not so a cyclic system contradicting the second law of
thermodynamics would be realizable). The composition of the vapour,
however, would not be the same as that of either layer. As the
distillation proceeded one layer would diminish more rapidly than the
other until only the latter would remain; this would then distil as a
completely miscible mixture.

The distillation of completely miscible mixtures is the most common
practically and the most complex theoretically. A coordination of the
results obtained on the distillation of mixtures of this nature with
the introduction of certain theoretical considerations led to the
formation of three groups distinguished by the relative solubilities